Audit ESLint rules and align with JS coding standards

[westonruter]

I just noticed that the one-var rule was incorrectly disabled, and fixed in #188.

We need to do an audit of the JS coding standards and make sure the ESLint config is aligned.

[westonruter]

I'm assuming this will just defer to https://github.com/WordPress-Coding-Standards/eslint-config-wordpress

Any changes to the ESLint config should be opened as pull requests to that repo.